1. Field of the Invention
This invention is related to providing higher quality digital image capture. Specifically, the invention provides a method and apparatus for reducing row reset noise in image sensors.
2. Description of Related Art
Image sensor circuits are used in a variety of different types of digital image capture systems, including products such as scanners, copiers, and digital cameras. The image sensor is typically composed of an array of light-sensitive pixels that are electrically responsive to incident light reflected from an object or scene whose image is to be captured.
The performance of an image capture system depends in large part on the sensitivity of each individual pixel in the sensor array and its immunity from noise. Pixel sensitivity is defined here as being related to the ratio of a change in the pixel output voltage to the photogenerated charge in the pixel. Noise here is defined as small fluctuations in a signal that can be caused by a variety of known sources. An image sensor with increased noise immunity yields sharper, more accurate images in the presence of environmental and other noise.
Improving the sensitivity of each pixel permits a reduction in exposure time which in turn allows the capture of images at a greater rate. This allows the image capture system to capture motion in the scene. In addition to allowing greater frame rate, higher pixel sensitivity also helps detect weaker incident light to capture acceptable quality images under low light conditions.
One way to increase pixel sensitivity is to increase the efficiency of the photodiode by changing the photodiode responsitivity characteristics. Doing so, however, can require deviating from a standard metal oxide semiconductor (MOS) integrated circuit fabrication process, thereby increasing the cost of manufacturing the image sensor circuit.
As stated above, integrated circuit imaging devices include an array of light detecting elements interconnected to generate analog signals representative of an image illuminating the device. Within such an integrated circuit, each complementary metal oxide semiconductor (CMOS) image sensing element contained in the integrated circuit contains a photodiode or phototransistor as a light detecting element. In one example, charge is collected in accordance with the intensity of light illuminating the photodiode or phototransistor. By storing charge, an analog signal is thus generated having a magnitude approximately proportional to the intensity of light illuminating the light detecting element.
During operation, each pixel has a photo-sensitive diode that is first reset by placing a charge across the photodiode. Then, the photodiode is exposed to incident light, which causes the charge stored on the photodiode to be dissipated in proportion to the intensity of the incident light. After a predetermined time period during which the photodiode is exposed to the incident light and charge is allowed to dissipate from the photo diode (i.e., the "integration" time), the charge at the node of the photodiode is read out. This valve is sampled on to a capacitor by opening a switch (i.e., a "SAMPLE" transistor).
When the time arrives to read-out the charge on the capacitor, an ADDRESS is provided to select the charge on the capacitor for read-out. After the charge on the capacitor has been read-out, the photodiode is reset by asserting a RESET signal to a reset transistor and the reset potential that is distributed across the photodiode is read-out. The amount of incident light that is detected by the photodiode is computed by subtracting the integrated voltage from the reset voltage level. This capture technique described above is known as "un-correlated double sampling," and serves to eliminate some fixed noise sources from each photodiode capture.
When determining the amount of light detected by the photodiode, noise that is generated by the switching of the reset transistor is captured during the reset of the photodiode. In addition, due to fluctuations in the power supply voltage, the reset level varies between resets. Thus, the "noise" present in the power supply also affects the reset level. As entire rows of photodiodes are reset at the same time, the noise generated affects all the photodiodes in a row. This results in each row having a different variation in noise that appears as row noise in an image. It is desirable to be able to eliminate the noise which is generated by the reset of the photodiode.
It is to be noted that although a specific architecture has been provided to describe the deficiencies in the prior art, architectures which have not been described can contain the same deficiencies. Thus, the problems described above can occur in all circuits that uses a different reset level from the level at which the photodiode begins to discharge.
It is therefore desirable to have a method and apparatus of using current pixel designs to achieve improved sensitivity and noise performance using electrical circuitry available with standard MOS fabrication processes.